I can't speak to the other clubs, but I think there will be a fair bit of trail and error with this this year!
At WORM we'd probably plan to allow all classes (2wd, 4wd & trucks) to run on all our race days...if we can! But on regional days entry priority goes to the class that's nominated.
As long as overall numbers on the day don't mean the day runs too long, then we'll be fine, but if we run into timing issues we may need to limit/cut the non nominated classes.
First step would be ask people who try to book into more than 1 class to drop to one class for the day.
If that doesn't reduce the heat listing sufficiently, then we'd need to cut secondary classes in some way, but we'd do everything we can to make sure everyone who turns up to race gets a race if possible.
Having said that, as long as we start in good time, I see no reason to need to do that. My experience of nationals sees they fit in 120 racers, 2x3min practices, 4 Qualifiers, single leg finals, and 3 leg A finals in their schedule, but they do start booking in at about 7am, practice at 8am and finish at about 6pm!!! We shouldn't have those numbers here, and we'd like a shorter day than that I suspect, so we'd look at something more reasonable.
Not sure if the region has a fixed schedule or if they let the host clubs control their days, but I'm fairly sure that until we get a feel for the numbers entering this year, we won't really know!
My guess...2wd days could get close to the wire on timings, but should be do'able on squeezing in other classes with a timely start time. 4wd days we should be fine, I suspect the entry will be lower this first year, so I don't foresee it being a major issue running other classes...as long as the 2wd's don't all come expecting to race as well!!!
I'd hope people coming to regionals aren't interested in running 2 (or more!!) classes on those days so they can focus on their primary class!